why did penny and lenard brek up.

So Leonard’s never been in a really serious relationship, so when he realized he was in love with Penny, he told her that. Penny, on the other hand, has been in a lot of relationships, and she’s said “I love you” before, too early in the relationship, and it never ended well so she basically told him she wasn’t sure if she could say it to him. (Seriously, writers, sitcoms are not for scarred pasts ruining the present, that's for DRAMAS YOU IDIOTS!) Then Wil Wheaton made up another story, Mee Maw Died Part II, and told Penny that it would be kinder to break up with Leonard now instead of chancing her not loving him (she said she wasn’t sure and Leonard and her originally decided to stay together and see what happens, since he’s liked her longer than him and all that) because it would hurt him less in the long run. So she ended it with him to make it easier long term on him.

Mainly cause Leonard told Penny he loves him and Penny thought it was to soon

After having sex, leonard told penney he loves her, she said "Thank you". Leonard got mad a Will Whedon, convinced penney into breaking up with her

This happened on Season 3 episode 18 or 19

It was because they weren't on the same level. Leonard told Penny he loved her and Penny didn't say it to him, which upset Leonard. In the end, with a bit of hint from Will Weaton, Penny broke up with Leonard.

The broke up because penny lied about having a college degree and told Sheldon about it.Because of his inability to keep a secret,he mistakenly told leonard.

Because the plot called for it.